The present invention relates to the production of crude oil wherein beam pumping units are used to actuate downhole pumps to lift the crude to the surface. Sucker rods are used to couple the beam pumping unit at the surface to the reciprocating pump located at the bottom of the well. Sucker rods are conventionally provided with male threads at each end and are coupled together by coupling units to form a continuous rod from the surface to the bottom of the well. Normally, the sucker rods are made of steel and provided with suitable threaded ends and flat sections to permit the use of wrenches in coupling the rods together. While steel rods are satisfactory, in some installations it would be desirable to use rods formed from non-corroding materials. This is especially the case when producing wells that contain sour crude, that is, crude having a high sulfur content that corrodes the rods and increases the maintenance required for the pumping unit.
In order to overcome the problem of corrosion in sour wells it has been suggested that fiberglass rods be used. Fiberglass rods consist of a thermosetting resin, either a polyester type resin or an epoxy type resin and reinforcing fiber materials. The most often used fiber reinforcing material is continuous lengths of glass fibers since they provide high strength at a relatively low cost. Other fiber such as graphite or boron could also be used but their high cost results in a relatively high priced sucker rod. Fiber reinforced plastic sucker rods are normally formed by extruding a continuous length of the fiber filament reinforcing material impregnated with a suitable thermosetting resin. The rods are made in relatively long lengths and then trimmed to the desired overall length and provided with threaded end fittings that are normally attached to the rod by an adhesive. The end fittings are made from steel or similar metals to withstand the wear of making up the rod string and disassembling it. The adhesive is normally epoxy and after the epoxy has cured, the rods are test pulled to some preset limit, normally one-half their ultimate strength, to ensure that the threaded ends are firmly attached to the rod. In the past the sole criteria for these tests has been either the rod failed if the end separated from the rod, or it passed the test. No attempt was made to evaluate the quality of the rod or the integrity of the joint between the end fitting and the rod body. This resulted in a large number of fiber reinforced resin type rods passing the initial test, yet failing in service by pulling off the end fitting or failure of the fiber reinforced plastic laminate itself.